This invention relates generally to color imaging and more particularly to the use of plural exposure and development steps for such purposes.
Several different methods exist for printing multi-color images. One method of printing in different colors is to uniformly charge a charge retentive surface and then optically expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop (REaD) process may be repeated to subsequently develop images of different colors in superimposed registration, whereby a full color image is created on the surface before being subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). The images may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
During the REaD image on image process of creating multi-color images, several critical issues must be addressed in the attempt to provide optimum conditions for the development of subsequent color images onto previously developed color images. Specifically, the voltages among previously toned and untoned areas of the photoreceptor must be leveled during a recharge step, so that subsequent exposure and development steps are effected across a uniformly charged surface. The greater the difference in voltage between those image areas of the photoreceptor previously subjected to a development and recharge step, and those bare non-developed, untoned areas of the photoreceptor, the larger will be the difference in print quality between these areas.
Another issue to be addressed with the image on image color image formation process is the need to substantially reduce, or eliminate, the residual charge that exists across the toner layer of a previously developed area of the photoreceptor before the subsequent exposure and development of the next color image thereon. Color quality is severely threatened by the presence of the toner charge and the resultant voltage drop across the toner layer. The residual toner voltage (V.sub.t) prevents the effective voltage above any previously developed toned areas from being re-exposed and discharged to the same level as neighboring bare photoreceptor areas which have been exposed and discharged to the actual desired voltage levels. The residual voltage associated with a toned image can be responsible for color shifts, increased moire, increased color shift sensitivity to image misregistration and motion quality, and loss in latitude affecting many of the photoreceptor subsystems. These problems become increasingly severe as additional color images are subsequently exposed and developed thereon. Thus, it is ideal to reduce or eliminate the residual toner voltage of any previously developed toned images.
Additional residual problems may occur while attempting to achieve both voltage uniformity and elimination of the residual toner voltage. For example, during the recharge step, if some or all of the potential associated with the previously developed image is raised to a high level relative to subsequently exposed areas, toner spray or spread is likely to occur along the edges of the developed image in areas where the edges of the prior developed image align but do not overlap with the edges of a subsequent image. Conversely, if during the attempt to avoid toner spread, the potential associated with the developed image is brought to a lower level during recharge, whereby some or all of the toner charge is reversed in polarity, a different problem of a serious nature develops. Since the prior toner image is now predominantly of an opposite polarity to both the background bare areas and the incoming color toner to be developed thereon, an interaction occurs among these three separate and distinctly charged regions. Particularly, the reverse polarity toner image is attracted to the background areas and the toner of the incoming color image, causing the positively charged toner of the first image to splatter into neighboring bare background regions at the edges of the prior image. This occurrence has been titled the "under color splatter" defect (UCS) and is the cause for the unwanted blending of colors and the spreading of colors from image edges into background areas. The UCS defect is apparent where the prior image overlaps with the subsequent image.
In an attempt to avoid both the above mentioned problems of toner spray and UCS occurrence while realizing a reduction in V.sub.t, it has been taught in the prior art to neutralize the charge of the toner layer. For example, a recharge method disclosed in copending application for U.S. patent entitled "Method and Apparatus for Reducing Residual Toner Voltage", Ser. No. 08/347,616, filed by a common assignee as the present application, discloses a recharge method wherein a voltage sensitive recharge device used for the recharging steps during a color image formation, whose graph of the output current (I) to the charge retentive surface as a function of the voltage to the charge retentive surface (V) has a high (I/V) slope. The high I/V slope recharge device disclosed having an AC voltage supplied thereto, enables an extended time for neutralization to occur at the top of the toner layers. However, the amount of residual voltage V.sub.t reduction that can be realized is limited in this system.
A recharge method disclosed in copending application for U.S. patent entitled "Split Recharge Method and Apparatus for Color Image Formation", Ser. No. 08/347,617, filed by a common assignee as the present application, discloses a recharge method wherein a corona generating apparatus recharges a charge retentive surface having at least one image developed thereon, to a predetermined voltage. A first corona generating device delivers a direct current to recharge the charge retentive surface to a higher absolute potential than the predetermined potential, which enables more opposite polarity charge to be delivered to the toner image by a second corona generating device during the neutralization process. The second corona device subsequently delivers an alternating current to the photoreceptor and toner image, to recharge the charge retentive surface to the predetermined potential and substantially neutralize the electrical charge associated with the developed image. The split recharge method disclosed in Ser. No. 08/347,617 thereby successfully enables voltage uniformity between toned and untoned regions, while substantially eliminating the residual voltage associated with the previously toned image. However, as the first (non-black) color toner deposited during the image on image process undergoes split recharge, i.e. a negative overcharging step and an AC charging step, from one to three times, a substantial amount of negative charge is driven through this first layer.
During corona charging of a toner image on a charge retentive surface, e.g. a photoreceptor belt, negative corona ions tend to drive down towards the bottom of the toner layer, and positive ions tend to stay at the top of the layer. Thus, negative charging of a toner layer causes negative charges to be forced to the bottom of, in particular, the first deposited color toner, and onto the photoreceptor surface, thereby causing potential difficulty during the attempt to transfer the toner image from the photoreceptor. It is therefore desirable to keep the particle charge level as low as possible at every stage of the image formation process.
Based on the foregoing, a highly reliable and consistent manner of recharging the photoreceptor to a uniform level is needed, whereby the residual voltage on previously toned areas is minimized and minimal negative charge is exerted on the toner particles.
The following references may be found relevant to the present disclosure.
Application for Japanese Patent No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sep. 4, 1991, assigned to Matsushita Denki Sangyo K. K. discloses a color image forming apparatus wherein a first and second charging device are used to recharge a photoconductor carrying a first developed image, before exposure and development of a subsequent image thereon. The potential of the photoconductor is higher after passing the first charging device than after passing the second charging device. This reference teaches that the difference in voltage applied by the first and second charging devices to the toner image and photoreceptor surface is set to a relatively high level, to insure that the polarity of the toner image is reversed after passing and having been charged by both devices, to reduce the residual charge in the image areas, and also to prevent toner spray during the exposure process. However, as previously discussed, the presence of reversed polarity toner at the top of the toner layer causes the serious defect of under color splatter.
U.S. Pat. No. 4,791,452 relates to a two-color imaging apparatus wherein a first latent image is formed on a uniformly charged imaging surface and developed with toner particles. The charge retentive surface containing a first developed or toned image, and undeveloped or untoned background areas is then recharged by a scorotron charging device prior to optically exposing the surface to form a second latent electrostatic image thereon. An electrical potential sensor detects the surface potential level of the drum to ensure that a prescribed surface potential level is reached. The recharging step is intended to provide a uniformly charged imaging surface prior to effecting a second exposure.
U.S. Pat. No. 4,819,028 discloses an electrophotographic recording apparatus capable of forming a clear multicolor image including a first visible image of a first color and a second visible image of a second color on a photoconductive drum. The electrophotographic recording apparatus is provided with a conventional charger unit and a second corona charger unit for charging the surface of the photoconductive drum after the first visible image is formed thereon so as to increase the surface potential of the photoconductive drum to prevent the first visible image from being mixed with a second color and also from being scratched off from the surface of the photoconductive drum by a second magnetic brush developing unit.
U.S. Pat. No. 4,761,669 relates to creating two-color images. A first image is formed using the conventional xerographic process. Thus, a charge retentive surface is uniformly charged followed by light exposure to form a latent electrostatic image on the surface. The latent image is then developed. A corona generator device is utilized to erase the latent electrostatic image and increase the net charge of the first developed image to tack it to the surface electrostatically. This patent proposes the use of an erase lamp, if necessary, to help neutralize the first electrostatic image. A second electrostatic image is created using an ion projection device. The ion image is developed using a second developer of a different color.
U.S. Pat. No. 4,033,688 discloses a color copying apparatus which utilizes a light-lens scanning device for creating plural color images. This patent discloses multiple charge/expose/develop steps.
U.S. Pat. No. 4,833,503 discloses a multi-color printer wherein a a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image by a corona generating unit and an erase lamp.
U.S. Pat. No. 5,208,636, discloses a printing system wherein charged area images and discharged area images are created, the former being formed first and the latter being proceeded by a recharging of the imaging surface.
U.S. Pat. No. 5,241,356 discloses a multi-color printer wherein charged area images and discharged area images are created, the former being formed first, followed by an erase step and a recharge step before the latter is formed. An erase lamp is used during the erase step to reduce voltage non-uniformity between toned and untoned areas on a charge retentive surface.
U.S. Pat. No. 5,258,820 discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development of a charged area (CAD), and a prerecharge corona device is used following development of a discharged area (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
Copending application for U.S. Pat. Ser. No. 08/1346,708, filed by a common assignee as the present application, discloses a corona recharge device for recharging the photoreceptor containing at least one previously developed color image, to a voltage level intermediate to the background areas and the image areas, to keep wrong-charge toner developed in the background areas at a charge level distinct from the toner developed in the image areas so that the wrong-charge background toner does not transfer to a support substrate with the image.
A number of commercial printers employ the charge/expose/develop/recharge imaging process. For example, the Konica 9028, a multi-pass color printer forms a single color image for each pass. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC1 machine employs an AC corona discharge device prior to recharge.